The invention relates to a control circuit for measuring the distance between a workpiece and an electrode which is formed as inductance and is part of a resonant circuit as a frequency-determining element whereby the frequency variations of the resonant circuit for producing voltage and/or current changes which are proportional to a distance are connected to a measuring circuit.
Devices have been long known which measure by capacitive means the distance between a tool and a workpiece and as a result make it possible to keep this distance constant via a control device. For example, electrodes are used in flame cutting machines which perform cutting procedures with acetylene gas which are attached to the tool and are arranged opposite the plate to be worked on in such a way that between the electrode and plate a distance remains which is regulated at a certain capacitance. Measuring the capacitance takes plate here electronically; a signal is transmitted from the output of the measuring instrument to the control device of the drive motor which automatically keeps the distance constant between workpiece and tool.
Such devices have also stood the test thus far in the use of plasma cutting torches. However, recently plasma cutting torches are used which operate with an additional water jacket which encloses the plasma arc like a jacket and, therefore, concentrates vapors and other polluting effects on the cutting procedure itself. The water jacket in such instruments is produced with a diameter of about 50 mm immediately above the arc electrode, concentric to the plasma arc, and strikes the workpiece at a right angle so that it consistently encloses the entire cutting area. When the water strikes, a water puddle several mm thick is formed on the workpiece, for example, a steel plate. This water is conductive and might be incorrectly evaluated by a capacitive electrode as the upper surface of the workpiece. As a result, the signal might be transmitted by the output of a capacitive distance measuring instrument, which might result in too great a distance between torch and workpiece surface. Capacitive concentric electrodes are, therefore, rejected for scanning in water plasma cutting instruments.
It was also already proposed to measure the distance between torch and workpiece inductively by using several coils arranged around the torch. Each of these coils is wired here as a frequency-determining element in a resonant circuit which is varied in frequency by an inductivity change of the coil as a result of a change in distance to the tool so that from the frequency variation, appropriate measuring signals can be derived in an in itself known way. The evaluation of such frequency variations, for example, in discriminator circuits or band selectors has been known and in use for a long time and requires no further explanation here.
In addition to relatively high mechanical effort because of the arrangement of at least three or four coils spaced around the torch, the inductive measuring circuits described here have the significant disadvantage that the circuit inductances are fully exposed to the influence sphere of the workpiece and possible disturbance variables such as heat from the plasma beam and the influence of the cooling water and that the distances between these inductances must, therefore, be large as a result of which control accuracy is reduced. The use of inductances for measuring distances, especially in applications requiring accurate distance regulation such as plasma cutting torches was, therefore, thus far not possible.